The present invention relates to the field of optical fiber transmission and more specifically to the compensation of cumulated chromatic dispersion and cumulated chromatic dispersion slope in optical fiber transmission systems.
For optical fibers, the index profile is generally qualified in relation to the graph of the function which associates the refractive index with the fiber radius. Conventionally the distance r to the center of the fiber is shown along the abscissa and the refractive index difference with the index of the fiber cladding is shown along the ordinate axis. The index profile is therefore referred to as “step”, “trapezoidal” or “triangular” profile for graphs having the respective shapes of a step, trapezoid or triangle. These curves are generally representative of the theoretical or set profile of the fiber, the manufacturing stresses of the fiber possibly leading to a substantially different profile.
An optical fiber conventionally consists of an optical core whose function is to transmit and possibly to amplify an optical signal, and of an optical cladding whose function is to confine the optical signal within the core. For this purpose, the refractive indexes of the core nc and of the outer cladding ng are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber decomposes into a fundamental mode guided in the core and into secondary modes guided over a certain distance in the core-cladding assembly.
In new, high bit-rate, wavelength multiplexed transmission systems it is advantageous to manage chromatic dispersion, in particular for rates of 10 Gbits/s or higher. The objective, for all multiplex wavelength values, is to achieve a cumulated chromatic dispersion that is substantially zero on the link, in order to limit pulse broadening and resulting interference. “Cumulated chromatic dispersion” relates to the integral of local chromatic dispersion over fiber length; chromatic dispersion being constant, the cumulated chromatic dispersion is equal to the product of chromatic dispersion multiplied by the length of the fiber. A cumulated value of a few dozen ps/nm for dispersion is generally acceptable at 40 Gb/s, of a few hundred at 10 Gb/s. It is also of advantage, in the vicinity of the wavelengths used in the system, to avoid zero values of local chromatic dispersion for which non-linear effects are stronger. Finally, it is also of advantage to limit the cumulated chromatic dispersion slope over the multiplex range so as to avoid or limit distortions between the multiplex channels. This slope is conventionally the derivate of the local or cumulated chromatic dispersion with respect to the wavelength.
As line fibers, for optical fiber transmission systems, single-mode fibers are conventionally used (SMF) or Non-Zero Dispersion Shifted Fibers (NZDSF+). NZDSF+ fibers are fibers having a non-zero, positive chromatic dispersion for the wavelengths at which they are used, typically around 1550 nm. For these wavelengths, these fibers have low positive chromatic dispersion, typically lower than 10 ps/(nm.km) at 1550 nm, and a positive local chromatic dispersion slope of between 0.04 and 0.1 ps/(nm2.km).
To compensate chromatic dispersion and chromatic dispersion slope in SMF or NZDSF+ fibers used as line fibers, short lengths of Dispersion Compensating Fiber can be used (DCF); said fiber then has a negative chromatic dispersion and a negative chromatic dispersion slope. For the choice of DCF fiber, it is generally sought that the ratio of chromatic dispersion over the dispersion slope of the compensating fiber is substantially equal to that of the line fiber. This ratio is designated by the abbreviation DOS for Dispersion Over Slope ratio.
U.S. Pat. No. 5,568,583 or U.S. Pat. No. 5,361,319 describe DCF fibers for compensating the chromatic dispersion of SMF fibers, and EP-A-1 067 412 describes a DCF fiber for compensating the chromatic dispersion of NZDSF fibers. These known DCF fibers, at a wavelength of 1550 nm, exhibit a negative chromatic dispersion and a negative chromatic dispersion slope.
Optical systems that are wavelength multiplexed, called Wavelength Division Multiplexing (WDM), generally consist of a concatenation of line fiber sections—SMF, NZDSF or other—with dispersion compensation modules inserted between the line fiber sections and comprising DCF rolled sections. The manner in which the dispersion compensation modules are distributed along the transmission line is called dispersion management; the objective of this management is to limit both non-linear effects and cumulated end-of-line dispersion. It is always sought, at the end of the line, to achieve a low cumulated chromatic dispersion and a zero cumulated dispersion slope.
In this context, by “transmission line section” is meant part of an optical transmission system linking a transmitting element to a receiving element, these elements possibly being located at the end of the line or in nodes of the optical system. A line section therefore comprises one or more concatenated line fiber sections and one or more sections of dispersion compensating fiber distributed between the sections of the line fiber. The line fiber sections usually generate positive chromatic dispersion with a positive chromatic dispersion slope, whereas sections of compensating fiber generate negative chromatic dispersion with a negative chromatic dispersion slope. In the event of overcompensation, the line section will therefore exhibit negative cumulated chromatic dispersion and negative chromatic dispersion slope which it is necessary to offset in order to arrive at zero dispersion at a node entry or at the end of the line.
It is sometimes of advantage to insert overcompensation along the transmission line, for example to limit non-linear effects in the line fiber. It has also been found that overcompensation of chromatic dispersion reduces the error rate at the receivers. For example, the article ‘Investigation of Advanced Dispersion Management Techniques for Ultra-Long Haul Transmissions” by J.-C Antona, M. Lefrancois, S. Bigo and G. Le Meur, presented in September 2005 to the ECOC'05 Conference (European Conference for Optical Communications) indicates that over-compensation during transmission, illustrated in the article by a residual dispersion per subdivision or per negative line fiber section, makes it possible to improve the performance of WDM systems at 10 Gb/s. However, at line end and/or at each node of the transmission system, the cumulated chromatic dispersion must be restored to zero or slightly positive. Yet, if the optical signal has been overcompensated, at the end of the line, the chromatic dispersion and the dispersion slope will be negative; it is then necessary, in order to offset this overcompensation, to use a fiber end having positive dispersion and a positive dispersion slope. For this purpose, sections of Standard SMF fibers are often used (SSMF) or Pure Silica Core Fibers (PSCF).
The major drawback with the use of a SSMF section to offset over-compensation is that SSMF induces high losses with respect to the quantity of dispersion produced. This characteristic is generally determined by what is called the “Figure of Merit” (FOM). FOM is defined as the ratio of chromatic dispersion D, in absolute value, to the attenuation of the signal in dB/km. For a SSMF fiber, the FOM value is in the order of 85 ps/nm/dB. PSCF fibers induce fewer optical losses and have a FOM value in the order of 125 ps/nm/dB, but they are costly.
U.S. Pat. No. 6,724,964 describes optical fibers having index profiles such that the fibers, for a high order propagation mode, exhibit positive chromatic dispersion. The profiles described in this application generate positive chromatic dispersion and a positive, zero or negative dispersion slope. The fibers described in this document are used to compensate the dispersion of line fibers having negative chromatic dispersion. The dispersion values of the described fibers are very high, in the order of 500 ps/nm/km at 1550 nm. In addition, the fibers described in this document do not exhibit a positive chromatic dispersion slope over the entire spectral band used; in particular at 1550 nm, the dispersion slopes of the illustrated fibers are all negative. The fibers described in U.S. Pat. No. 6,724,964 could not be used therefore to counter over-compensation of a line section having negative cumulated chromatic dispersion and negative chromatic dispersion slope.
U.S. Pat. No. 2003/0202761 describes a fiber which, for a propagation mode other than the fundamental mode, exhibits a positive chromatic dispersion and a negative chromatic dispersion slope. This compensation fiber is particularly suitable for compensating a line fiber having a negative chromatic dispersion with a positive chromatic dispersion slope, such as the fiber marketed by Corning under the trade name Corning LS®. The fiber described in U.S. Pat. No. 2003/0202761 is therefore not adapted for offsetting overcompensation of a line section having negative cumulated chromatic dispersion and negative chromatic dispersion slope.
There is therefore a need for a chromatic dispersion compensating fiber with which it is possible to offset overcompensation at the end of the line or on node entry in an optical system, and which has a FOM factor that is higher than that of a SSMF or PSCF fiber.